The invention relates generally to sensing ground faults in electrical equipment, such as motor drives. Particularly, this invention relates to modular line-to-ground fault identification techniques.
Many electrical devices, such as motor drives, often include converter circuitry for converting incoming AC power into DC power, and inverter circuitry for converting the DC power into controlled frequency AC power output. Output currents for three phases of the AC power may be sensed for monitoring and control functions, such as for motor control and protection. For example, a ground current may be determined by summing all three measured phase currents. When the sum of the three phase currents exceeds a threshold value (e.g., 20% of the rated current for the motor drive), a ground fault of the system may be considered to have occurred. Often, ground fault detection samples the sum of the three phase currents, filters the result, and generates an alarm when the threshold value is exceeded. The threshold value may be adjustable to low levels, but this method of ground fault detection is still somewhat sensitive to noise and feedback accuracy. Furthermore, the ratio between the fault current and the nominal current of the motor might be comparable to the signal-to-noise ratio of the current sensor used and, thus, the system may not detect a ground fault when it perhaps should. Therefore, there is a need for more reliable detection of ground faults.